cmake: update list of "advanced" variables

To hide them from the CMake GUI by default.

Closes #14540

cmake: more small tidy-ups

- prefix local variables with underscore and convert to lowercase.
- list variables accepted by `libcurl.pc` and `curl-config` templates.
- quote more stri

cmake: more small tidy-ups

- prefix local variables with underscore and convert to lowercase.
- list variables accepted by `libcurl.pc` and `curl-config` templates.
- quote more string literals.

Follow-up to 919394ee645be8cf92a96ff01140b4daf9cb21a5 #14450
Closes #14462

show more ...

cmake: add Linux CI job, fix pytest with cmake

- extend existing Linux workflow with CMake support.
Including running pytest the first time with CMake.

- cmake: generate `test

cmake: add Linux CI job, fix pytest with cmake

- extend existing Linux workflow with CMake support.
Including running pytest the first time with CMake.

- cmake: generate `tests/config` and `tests/http/config.ini`.
Required for pytest tests.
Uses basic detection logic. Feel free to take it from here.
Also dump config files in a CI step for debugging purposes.

- cmake: build `tests/http/clients` programs.

- fix portability issues with `tests/http/clients` programs.
Some of them use `getopt()`, which is not supported by MSVC.
Fix the rest to compile in CI (old-mingw-w64, MSVC, Windows).

- GHA/linux: add CMake job matching an existing autotools one.

- GHA/linux: test `-DCURL_LIBCURL_VERSIONED_SYMBOLS=ON`
in the new CMake job.

- reorder testdeps to build server, client tests first and then
libtests and units, to catch errors in the more complex/unique
sources earlier.

- sort list in `tests/http/clients/Makefile.inc`.

Closes #14382

show more ...